U.S. patent number 8,075,751 [Application Number 12/252,692] was granted by the patent office on 2011-12-13 for water chlorinator having dual functioning electrodes.
This patent grant is currently assigned to Finnchem USA, Inc.. Invention is credited to Dennis Frederick Dong, Yuanwu Xie.
United States Patent |
8,075,751 |
Xie , et al. |
December 13, 2011 |
Water chlorinator having dual functioning electrodes
Abstract
A water chlorinator includes an aqueous chloride ion source; and
a pair of dual functional electrodes configured to electrolyze the
aqueous chloride ion source, each one of the pair of dual
functional electrodes comprising a titanium substrate and a mixed
metal oxide coating deposited thereon and consisting essentially of
ruthenium oxide and titanium oxide having a molar ratio of 5:95 to
25:75, respectively.
Inventors: |
Xie; Yuanwu (Columbia, SC),
Dong; Dennis Frederick (Columbia, SC) |
Assignee: |
Finnchem USA, Inc. (Eastover,
SC)
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Family
ID: |
42107778 |
Appl.
No.: |
12/252,692 |
Filed: |
October 16, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100096260 A1 |
Apr 22, 2010 |
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Current U.S.
Class: |
204/278.5;
204/275.1 |
Current CPC
Class: |
C02F
1/4674 (20130101); C02F 2103/42 (20130101); C02F
2201/4613 (20130101); C02F 2001/46142 (20130101); C02F
2001/46157 (20130101) |
Current International
Class: |
C25B
9/06 (20060101); C25B 11/06 (20060101); C25B
11/08 (20060101) |
Field of
Search: |
;204/275.1,278.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2006005836 |
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Jan 2006 |
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WO |
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2007022572 |
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Mar 2007 |
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WO |
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Other References
Leiderbach, Thomas A., "Metal Anodes" Kirk-Othmer Encyclopedia of
Chemical Technology, Copyright John Wiley & Sons, Inc.,
Electrode Corporation, vol. 15, pp. 1-17. cited by other .
Wojtowicz, John A., "Water Treatment of Swimming Pools, Spas, and
Hot Tubs" posted ECT (online) Dec. 4, 2000, pp. 2-33. cited by
other .
Arikawa, T., et al. "Simultaneous determination of chlorine and
oxygen evolving at RuO2/Ti and RuO2-TiO2/Ti anodes by differential
electrochemical mass spectroscopy", 1988 Chapman & Hall;
Journal of Applied Electrochemistry 28 (1998), pp. 511-516. cited
by other .
Burrows, I.R., et al. "Chlorine and Oxygen Evolution on Various
Compositions of RuO2/TiO2 Electrodes" Electrochimica Acta, Pergamon
Press, Ltd., 1978, vol. 23., pp. 493-500. cited by other .
Spasojevic, M.D. et al. "Optimization of Anodic Electrocatalyst:
RuOx/TiO2 on Titanium" J. Res. Inst. Catalysis, Jokkaido Univ.,
vol. 31, Nos. 2/3, pp. 77 to 94 (1983). cited by other.
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Primary Examiner: Bell; Bruce
Attorney, Agent or Firm: Thomas, Kayden, Horstemeyer &
Risley, LLP
Claims
What is claimed is:
1. A water chlorinator, comprising: an aqueous chloride ion source;
and a pair of dual functional electrodes configured to electrolyze
the aqueous chloride ion source, each one of the pair of dual
functional electrodes comprising a conductive substrate and a mixed
metal oxide coating deposited thereon consisting essentially of
ruthenium oxide and titanium oxide having a molar ratio of 5:95 to
25:75, respectively.
2. The water chlorinator of claim 1, wherein the molar ratio of the
ruthenium to the titanium is 15:85 to 20:80, respectively.
3. The water chlorinator of claim 1, wherein the molar ratio of the
ruthenium to the titanium is 15:85, respectively.
4. The water chlorinator of claim 1, wherein the ruthenium in the
mixed metal oxide coating is at a loading of 5 to about 20
g/m.sup.2.
5. The water chlorinator of claim 1, wherein the ruthenium in the
mixed metal oxide coating is at a loading of 10 to about 15
g/m.sup.2.
6. The water chlorinator of claim 1, wherein the dual functional
electrode is a mesh screen.
7. The water chlorinator of claim 1, wherein the dual functional
electrode is a plate.
8. The water chlorinator of claim 1, wherein the conductive
substrate is selected from the groups consisting of titanium, lead,
tantalum, tungsten, molybdenum, vanadium, zirconium, and
niobium.
9. The water chlorinator of claim 1, wherein the conductive
substrate is titanium.
10. The water chlorinator of claim 2, wherein the conductive
substrate is titanium.
Description
BACKGROUND
The present disclosure generally relates to dual functioning
electrodes adapted for anodic and cathodic use for a reverse
current electrolytic chlorination apparatus such as may be desired
for treating pool water, spas, and the like.
Electrolytic pool chlorinators have evolved to overcome the
problems associated with chemical dosing of swimming pools, spas,
and the like to prevent the accumulation growth of algae and
bacteria therein. The electrolytic chlorinator generally includes
two spaced apart electrodes including an anode for oxidation of
chloride ions from, normally, sodium based chloride salts to
chlorine, which subsequently hydrolyzes in solution to form
hypochlorite; and a cathode for reduction of water to hydrogen.
Water to be treated is dosed with the chloride salts and flows
between the electrodes. The electrolytically generated chlorine and
hypochlorite act as the active ingredients to oxidatively destroy
bacteria and other harmful agents in the water.
One of the disadvantages associated with electrolytic disinfection
is the cost of the electrolytic cell, as well as the cost of
replacement electrodes, which can corrode, become fouled with scale
and the like or otherwise become inactivated over time. These costs
are primarily driven by the size of the electrodes, which are
typically constructed from titanium coated with platinum or
ruthenium. Electrodes having a surface area sufficient to generate
adequate chlorine levels represent a significant portion of the
cost of installing and maintaining an electrolytic disinfection
system. In addition, electrolytic cell life is limited due to the
current density through the cell over time.
In order to keep the electrodes clean and operating at maximum
efficiency, the electrolytic current fed to the chlorinator can be
configured with dual functional electrodes, wherein each electrode
can dually function as the anode or cathode depending on whether
the current flow is in the forward or reverse direction. This so
called current reversal or reverse polarity operation exchanges the
chemical reactions that occur on the respective electrodes and in
doing so cleans the electrode surface. If mineral deposits are not
removed from the chlorinator, the electrodes would soon cease to
function because the deposits would cause the unit to reach a
so-called "high voltage" cutoff, much like it does with current
electrolytic cells that have single functioning electrodes.
One such dual functioning electrode is based on a coating of
catalytic oxide mixture of ruthenium dioxide (RuO.sub.2) and
titanium dioxide (TiO.sub.2) deposited onto a conductive substrate
such as titanium. Based on its behavior as a continuous
(uni-functional) anode, this particular mixed metal oxide is
typically used at a mole ratio of about 40:60 to about 50:50
(RuO.sub.2:TiO.sub.2) formed on a titanium substrate. It is
generally known that the operating lifetime of the coating for
electrolytic applications depends to a large extent on the amount
of the coating applied to the substrate. The total amount of
ruthenium that is in a typical coating for electrolytic pool
chlorinators is about 20 to about 30 g/m.sup.2 as Ru metal,
application of which is generally provided by solvent coating
multiple layers, typically about 20 to 30 coats. At ruthenium
concentrations below 40 mole %, durability is known to
significantly decrease when analyzing its capability as a
continuous anode. For example, as discussed in an article entitled,
"Optimization of an Anodic Electrocatalyst: RuO.sub.2/TiO.sub.2 on
Titanium", to Spasojevic et al. (J. Res. Inst. Catalysis, Hoklaido
Univ. Vol. 31, Nos. 2/3, pp 77-94, 1983), when measuring the change
in anode potential as a function of time for chlorine evolution at
3 kA/m.sup.2, 80.degree. C. and constant brine concentration, it
was observed that durability was at a maximum at 40 mol % RuO.sub.2
and decreased rapidly below 20%.
However, ruthenium is relatively expensive and efforts have been
ongoing to reduce the amount of ruthenium used by use of less
expensive metals. Because of this issue with durability when
continuously functioning anodically without periodic reverse
polarity, prior attempts to reduce the amount of ruthenium because
of its expense have generally been directed to substitution of
ruthenium with other metals, e.g., tin.
Accordingly, there remains a need for improved dual functioning
electrodes that exhibit prolonged durability and use decreasing
amounts of ruthenium.
BRIEF SUMMARY
Disclosed herein is a water chlorinator comprising an aqueous
chloride ion source; and a pair of dual functional electrodes
configured to electrolyze the aqueous chloride ion source, each one
of the pair of dual functional electrodes comprising a titanium
substrate and a mixed metal oxide coating deposited thereon and
consisting essentially of ruthenium oxide and titanium oxide having
a molar ratio of 5:95 to 25:75, respectively.
The disclosure may be understood more readily by reference to the
following detailed description of the various features of the
disclosure and the examples included therein.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the FIGURES wherein the like elements are numbered
alike:
The FIGURE illustrates a partial sectional view of a water
chlorinator.
DETAILED DESCRIPTION
Disclosed herein are dual functioning electrodes for water
chlorinators. The dual function electrodes are formed of the same
material and are configured to function as both anodically and
cathodically depending on current polarization. In one embodiment,
the dual function electrode includes a coating containing a mixed
metal oxide consisting essentially of 5 to 25 mol % RuO.sub.2 with
the remainder TiO.sub.2 deposited onto a conductive substrate, and
in other embodiments, about 15 to 20 mol % RuO.sub.2 with the
remainder TiO.sub.2 and in still other embodiments, about 15 mol %
RuO.sub.2. The total amount of ruthenium (Ru) in the coating is
about 10 to 15 g/m.sup.2, which is about 50% less than that
previously used, thereby providing a significant commercial
advantage. By way of example, in one embodiment, the molar ratio of
ruthenium to titanium is 5:95 to 25:75 respectively; in other
embodiments, 15:85 to 20:80, respectively, and in still other
embodiments, the molar ratio is 15:85, respectively. As will be
described in greater detail herein, it has unexpectedly been found
that current efficiency and accelerated lifetime analysis for the
relatively low ruthenium coating as noted above, when exposing the
electrodes in a reverse current, was about the same as continuous
anode formed of titanium having a coating at a 30:70 ratio of
RuO.sub.2 relative to TiO.sub.2 (i.e., without reverse current). As
noted above, previous studies had shown that anodes having lower
RuO.sub.2 content (i.e., less than 20%) exhibited poor durability
relative to coatings with higher RuO.sub.2 content, i.e., greater
than or equal to 30 to about 40 mole %. Thus, these results were
surprising and unexpected.
Suitable conductive substrates include, without limitation,
titanium, lead, tantalum, tungsten, molybdenum, vanadium,
zirconium, niobium, and the like.
A portion of the electrode may have a coating either applied to it
or an additional strip of coated titanium may be spot welded to the
electrode. The coating on the titanium is composed of ruthenium and
titanium. The use of the same material makes it possible to use a
reversal of polarity of the power source to have a self-cleaning
effect and increase the lifetime of the cell before any maintenance
is needed. In all applications, the conductive metal base is
cleaned and free of oxide or other scale. This cleaning can be done
in any way, by mechanical or chemical cleaning, such as, by sand
blasting, etching, pickling or the like.
The ruthenium dioxide/titanium dioxide mixed metal oxide coating
may be applied in various ways, and to various forms of the
electrode including but not limited to, such as solid rolled
plates, perforated plates, slitted, reticulated, plates, mesh and
rolled mesh, woven titanium wire or screen, rods and bars and the
like. Application can be by chemi-deposition in the form of
solutions painted, dipped or sprayed on or applied as curtain or
electrostatic spray coatings, baked on the metal base, but other
methods of application, including electrophoretic deposition or
electrodeposition, may be used. Care must be taken that no air
bubbles are entrapped in the coating and that the heating
temperature is below that which causes warping of the base
material.
In one embodiment, the ruthenium dioxide and titanium dioxide mixed
metal oxide coatings are formed from chemical precursors that are
solvent coated onto the substrate. Multiple coatings are utilized
to provide a ruthenium content of about 10 to about 15 g/m.sup.2.
The coated substrate is then thermally treated to provide the
respective metal oxides. By way of example, RuCl.sub.3 is dissolved
in HCl and then dissolved in isopropanol together with TiCl.sub.4.
The solution can be coated onto the substrate and dried. Multiple
coatings are provided to obtain the desired ruthenium content. The
substrate is then thermally heated in a furnace or the like at a
temperature and for a period of time effective to thermally
decompose the salts and form the metal oxide coating. Suitable
ruthenium precursors include, without limitation,
Ru(NO)(NO.sub.3).sub.3, RuCl.sub.3.3H.sub.2O,
Ru(NH.sub.3).sub.6Cl.sub.3, RuCl.sub.3NO.xH.sub.2O and others.
Suitable titanium precursors include, without limitation,
Ti(OBu).sub.4, Ti(OEt).sub.4, Ti(OPr).sub.4, TiCl.sub.3,
C.sub.10H.sub.10TiCl.sub.2, and others. Other suitable precursors
will be apparent to those skilled in the art in view of this
disclosure.
The dual functional electrodes 102 as described above are disposed
within a water chlorinator as is generally shown in the FIGURE by
reference numeral 100. The dual functional electrodes are spaced
apart from one another and are in electrical communication with a
power source 108. Water to be treated is dosed with the chloride
salts and flows between the electrodes. The power source 108 is in
operative communication with a controller (not shown) configured to
periodically reverse polarity such that the electrodes dually
function for a selected period of time as an anode and then as a
cathode. Each dual functional electrode 102 includes a base
substrate 104 upon which a mixed metal oxide coating 106 consisting
essentially of ruthenium and titanium is disposed. The electrodes
102 are not intended to function as a continuous anode or
continuous cathode without current reversal since it is well known
that durability becomes an issue. Reversal of current can be
effected as frequently as necessary to maintain each surface
substantially free of both mineral and biological deposits.
With respect to continuous operation as a cathode, the low ratio
ruthenium coating is especially prone to spalling. When functioning
cathodically, the cathode reacts with water to produce hydrogen
atoms, which subsequently combine to form hydrogen gas. If the
cathode is run continuously for extended periods of time, the
hydrogen becomes absorbed and can react with the base substrate to
form hydrides thereof, e.g., titanium hydride. The formation of
hydrides and spalling of the coating are limiting variables
affecting durability of the electrode. Formation of the titanium
hydride, for example, would cause the catalytic coating to spall
and the electrode would not function any more as an anode when the
current is reversed.
The following examples are presented for illustrative purposes
only, and are not intended to limit the scope of the invention.
EXAMPLE 1
In this example, a dual functional titanium electrode having a
mixed metal oxide coating formed of ruthenium oxide and titanium
oxide at a molar ratio of 15:85, respectively, was prepared.
To 3.6 g of ruthenium chloride hydrate was added 6 mL of 37% HCl.
After stirring this mixture, 28.9 mL of orthobutyltitanate and 70
mL of n-butanol were added and the mixture was thoroughly mixed. A
titanium sheet of thickness 0.04 inches was etched for 30 minutes
in hot hydrochloric acid (20%) and then rinsed and dried in air.
The coating solution was brushed onto the sheet of etched titanium
and the coated titanium was then dried in air for 10 minutes. The
coated sample was then placed into a muffle furnace at 450.degree.
C. for 10 minutes. After the sample was removed from the oven and
allowed to cool, a second coat of the coating solution was applied
in the same way; the sample was dried and then baked. This
procedure was repeated multiple times over three days until the
sample had achieved a coating loading of 14.4 grams of ruthenium
per square meter of coated surface as measured by X-ray
fluorescence spectroscopy. Each day after eight to ten coatings,
there was an extended bake of one hour at 525.degree. C. The ratio
of titanium to ruthenium in the resulting coating was calculated to
be 85:15.
A 0.8'' diameter sample disc was then punched from the electrode of
Example 1. A solution was prepared containing 0.5 molar sodium
sulfate. The sample was placed into a tall form beaker with a
platinum counter electrode and a saturated calomel reference
electrode. The electrode sample was polarized alternately
anodically and cathodically on a four minute cycle time (two
minutes anodic, two minutes cathodic) using an Arbin Instruments
MSTAT potentiostat. In this way, the electrode acted alternately as
an anode and as a cathode, and different electrochemical reactions
occurred on the surface of the electrode under each different
condition. During operation as a cathode, the cathode potential
generally remained the same, and the potential returned to the same
value on each return to cathodic polarization. On anodic
polarization, the potential remained generally the same until,
after 69 hours, the anodic potential of the sample rose more than
two volts over the starting potential, indicating failure of the
coating. The sample surface showed small pockets of exposed
substrate, indicating delamination of the coating from the titanium
substrate. The unit lifetime was calculated to be 4.8 hours per
gram of ruthenium per square meter.
EXAMPLE 2
In this example, a dual functional titanium electrode having a
mixed metal oxide coating formed of ruthenium oxide and titanium
oxide at a molar ratio of 20:80, respectively, was prepared.
To 0.354 g of ruthenium chloride hydrate was added 6 mL of 37% HCl.
After stirring this mixture, 10.3 mL of titanium chloride (8.4%
TiCl.sub.3 in 30% HCl) and 40 mL of isopropanol were added and the
mixture was thoroughly mixed. A titanium sheet of thickness 0.04
inches was etched for 30 minutes in hot hydrochloric acid (20%) and
then rinsed and dried in air. The coating was applied as described
in Example 1 until the loading had reached 7.0 grams of ruthenium
per square meter of coated surface as measured by X-ray
fluorescence spectroscopy. The ratio of titanium to ruthenium in
the coating is calculated to be 80:20.
This sample was placed under an accelerated life test as explained
in Example 1. After 48.5 hours, the anodic potential of the sample
rose more than two volts over the starting potential, indicating
failure of the coating. The sample surface showed small pockets of
exposed substrate, indicating delamination of the coating from the
titanium substrate. The unit lifetime was calculated to be 6.9
hours per gram of ruthenium per square meter.
COMPARATIVE EXAMPLE 1
In this example, a dual functional titanium electrode having a
coating formed of ruthenium oxide and titanium oxide at a molar
ratio is 30:70, respectively, was prepared.
The electrode was prepared in accordance with Example 1 using the
following quantities to prepare the coating solution: 7.2 g of
ruthenium chloride hydrate, 6 mL of 37% HCl, 23.8 mL of
orthobutyltitanate and 70 mL of n-butanol. The coating was applied
as described in Example 1 until the loading had reached 28.5 grams
of ruthenium per square meter of coated surface. The ratio of
titanium to ruthenium in the coating is calculated to be 70:30.
This sample was placed under an accelerated life test as explained
in Example 1. After 69 hours, the anodic potential of the sample
rose more than two volts over the starting potential, indicating
failure of the coating. The sample surface showed small pockets of
exposed substrate, indicating delamination of the coating from the
titanium substrate. The unit lifetime is calculated to be 2.4 hours
per gram of ruthenium per square meter.
From comparison of the results of the preceding example 1 and
comparative example 1, it is evident that the same accelerated
lifetime can be obtained from the sample formed from the teachings
of this invention, with much less ruthenium, compared to the
coating formed according to the prior art for anodes for chlorine
evolution (not dual purpose). Thus, even though one half the amount
of the expensive precious metal, ruthenium, is employed, a useful
and long-life electrode for pool cell electrolysis has been
shown.
In order to compare all three samples in the different amounts of
ruthenium loading, one can compare the unit lifetimes, the lifetime
duration divided by the loading of the precious metal. The
following table indicates that the two examples of the present
invention from Examples 1 and 2 have greater unit lifetimes
compared to that for the coating with a coating ratio as
recommended by the prior art.
TABLE-US-00001 TABLE 1 Example % Ru in the Coating Unit Lifetime,
hr-m.sup.2/g 1 15 4.8 2 20 6.9 Comp. 1* 30 2.4 *comparative
example
Current efficiency tests of the samples made under the same
conditions as Example 1 and Comparative Example 1 were carried out
as follows. The electrodes were placed as anodes into a bath
containing a solution of 3.0 grams of sodium chloride per liter of
solution at pH about 7. During electrolysis, the oxygen gas
evolving from the anode was collected in a gas sample collection
tube. The competing reaction of chlorine evolution produced
chlorine gas, which dissolved and hydrolyzed in the solution to
produce sodium hypochlorite. The amount of oxygen gas evolved
within a set amount of time was compared with the amount of gas
evolved with the same experiment but with sodium sulfate
substituting for the sodium chloride in the solution. Using the
sample produced according to the control test of Comparative
Example 1, the current efficiency for chlorine was measured to be
61%. For the sample produced according to Example 1, the current
efficiency for chlorine was measured to be the same or marginally
better, 62%. Thus, the same or improved current efficiency for
chlorine evolution can be obtained from a sample made according to
the invention described here, even though only one half of the
expensive precious metal, ruthenium, has been used.
As shown, the current efficiency and accelerated lifetime was
unexpectedly similar to the comparative example, indicating that
robust performance can occur upon a reduced loading ratio of
ruthenium in the coating when cycling current flow direction.
This written description uses examples to disclose the invention,
including the best mode, and also to enable any person skilled in
the art to make and use the invention. The patentable scope of the
invention is defined by the claims, and may include other examples
that occur to those skilled in the art. Such other examples are
intended to be within the scope of the claims if they have
structural elements that do not differ from the literal language of
the claims, or if they include equivalent structural elements with
insubstantial differences from the literal languages of the
claims.
* * * * *